Very weird to revise for them.
They're multiple-choice reasoning tests. Verbal, numerical, non-verbal.
If it is the same CAT treats as the ones I am familiar with, you will get a rather complicated set of results. Basically, the CEM centre in Durham have a huge amount of data for these tests. They will compare what answers your child put in with the results of hundreds of thousands of other students who have since taken their GCSEs.
In the old days, you would then have received something like
English
4-A
27-B
52-C
17-D
Meaning that, of the previous data sets, of 100 kids who have the same answers your kid did, 4 got an A in GCSE English, 27 got a B, 52 got a C, etc etc and ad infinitum through all the subjects.
Basically, a spread of probability of a child's likely future attainment at GCSE.
I don't know how they've handled the massive changes in GCSE exams, mind. That must've been a headache for them.
I have absolutely no idea why the school are encouraging your child to revise. Who wants inflated target grades????